Picture 8. The source sends a beam through the mould to the detector
How do we measure the level?
The steel and the source
The steel flows through the mould. The radioactive source is mounted to the side of the mould in a lead box. It is a pencil source. A small window lets out a beam that spreads in a fan from the source.
The detector has a scintillator which is on the other side of the mould. It is tall and thin, meaning it will pick up radiation that has spread out from the source. When the steel is at the lowest level, most of the radiation hits the scintiallator. As the steel level rises, it interrupts more of the beam reducing the amount of radiation that reaches the detector.
The signal from the detector is fed into a computer which calculates the level of the steel. From this information, the flow of molten steel can be adjusted to compensate for any changes from the required depth.
Picture 9. A graph of the count rate. If the count rate drops, it might be due to noise rather than a rise in steel level.
The ionising radiation from any radioactive source is not given out at a constant rate. Therefore the count rate goes up and down, even if the level of steel is constant. We call these random fluctuations noise. The steel level has to change by about 10 mm before the system can be sure that the change in count is not due to random noise. In some applications, this is not acceptable.
Corus scientists are currently working on an improved system that can respond more quickly to smaller changes in the depth of steel.
Picture 10. A scintillator. The body of the scintillator is a single, pure crystal about 30 cm long - costing thousands of pounds.
Detecting the radiation
Gamma radiation is detected using a device called a scintillator. A typical scintillator is held in a cylinder about 30 cm long. Inside the tube, there is a single crystal. When the crystal absorbs some gamma radiation, it gives out a tiny pulse of light. This light pulse is picked up by a very sensitive detector which sends a pulse of electricity to the counter. Each gamma ray produces a single pulse and registers as a single count on the counter.
How much radiation is there?
It is important that the radiation is not a hazard to the operators. The source is held in a lead box with a window pointing towards the mould. The only significant radiation that escapes from the box passes through the mould and the steel and hits the detector. There is also a screen behind the detector to reduce the radiation to a safe level.